A mixer is a device used to combine two or more signals. For example, it is commonly used in radio frequency (RF) transmitters and receivers to down-convert signals from a high frequency (RF) to a lower frequency (IF) or vice versa using a local oscillator (LO). One type of mixer is referred to as a Gilbert cell and is implemented using transistors.
Mixers can have a variety of topologies. A single-balanced mixer up-converts or down-converts an input RF signal, but the LO signal still leaks through to the output. A double balanced mixer is provided with both polarities of a balanced signal and can therefore cancel out one of the signals you wish to cancel, typically the LO. In reality, however, even well designed mixers still have some LO leakage, also known as feedthrough.
One solution for minimizing leakage at the output of a mixer is adding a filter. However, this type of system requires a tradeoff between a higher frequency input signal and a lower frequency input signal. The higher frequency input signal results in a higher frequency output signal (given a fixed LO) and therefore a smaller filter with fewer elements. This is practical for on-chip realization, but is difficult on the digital-analog converter. On the other hand, a lower frequency input signal features a more difficult and impractical on-chip filter design.
A particular type of double balanced mixer, a double-balanced Gilbert cell, is shown in FIG. 1. Mixer 100 is shown configured for use as an up-converter, where a lower frequency (IF) is mixed with a LO signal to generate a higher frequency (RF) output. It includes six transistors that are field effect transistors (FETs) in a preferred embodiment but other switching components, such as bipolar junction transistors (BJTs) could also be used. The IF signal is connected to the gate terminal of transistor 102 and an inverse of the IF signal is connected to the gate terminal of transistor 104. The source terminals of transistors 102 and 104 are both connected to element 106, shown in FIG. 1 as a current source. Alternatively, element 106 could be a ground node if mixer 100 is being used as a down-converter.
The LO signal is connected the gate terminals of transistors 108 and 110 and an inverse of the LO signal is connected to the gate terminal of transistors 112 and 114. The source terminals of transistors 108 and 112 are both connected to the drain terminal of transistor 102. The source terminals of transistors 110 and 114 are both connected to the drain terminal of transistor 104. The output RF signal is provided at the drain terminals of transistors 108, 110, 112 and 114 at output 116.
A Gilbert cell mixer typically provides high LO signal leakage rejection however, prior art designs have been unable to completely eliminate LO signal leakage. One solution is to use a cancellation technique that externally splits the LO signal and combines it with the output signal of the mixer after passing it through an external phase shifter and a variable attenuator. This solution is difficult to implement on a single chip since there it requires a variety of components which are made using different manufacturing technologies. Further, the different routing paths for the LO signal (the mixer forms one path and the phase shifter/attenuator forms another path) often cause mismatched phase relationships between the two LO signals that result in imperfect LO signal leakage cancellation.
Thus, a need exists for a mixer circuit having a reduced LO feedthrough.